This invention relates to a circuit arrangement for processing colour television signals incorporated in a multi-mode type colour TV receiver which is capable of receiving TV signals processed by different television systems such as NTSC and PAL systems.
As is well known, three TV systems i.e., NTSC, PAL and SECAM are currently in use. Consequently, there is considerable commercial demand to develop a single colour television receiver, a so-called "a multi-mode type receiver" that can receive different systems of TV signals. This is because, although different countries employ different TV systems, recent progress in satelite colour television broadcasing and also the advent of video tape recorders have made international broadcasing and viewing practical.
In the conventional multi-mode type colour television receiver, independent signal processing circuits are assembled to process each TV system. However, in such a receiver, due to the great numbers of circuit elements employed in the receiver, power consumption increases, manufacturing costs increase, and product reliability deteriorates.
To mitigate these conventional drawbacks a multi-mode type receiver was recently developed in which the total number of circuit elements was reduced by introducing common use circuit portions as much as possible. For example, the known multi-mode type colour television receiver can process both NTSC and PAL TV signals.
Specific circuit portions for each system in the NTSC and PAL colour TV receivers will now be explained with reference to FIGS. 1 and 2.
FIG. 1 shows a circuit diagram of the known colour signal processing circuit of the NTSC colour television receiver. The numeral 11 denotes an input terminal for a chroma signal and the numeral 12 indicates a bandpass amplifier. The bandpass amplifier 12 includes an automatic colour control circuit (ACC) and a burst-gated amplifier; the former detects a level variation of the input signal so as to automatically maintain the output level constant, and the latter separates the input signal into the chroma signal component and the burst signal. The chroma signal separated in the bandpass amplifier 12 is input via a transmission path to a colour control circuit 14, which is amplified in accordance with auser's adjustment. The chroma signal derived from the colour control circuit 14 is supplied to a (B-Y) demodulator 15 and an (R-Y) demodulator 17. The burst signal separated in the bandpass amplifier 12 is supplied via a signal transmission path 18 to a hue control circuit 19. This hue control circuit 19 functions to correct the hue error caused by adverse influences on the signal during transmission in the signal path. This correction, too, is effected according to the user's adjustment. The burst signal which has been phase-adjusted in the hue control circuit 19 is supplied to a colour sync circuit 21 through a transmission path 20. The colour sync circuit 21 includes a colour reference subcarrier generator for generating a subcarrier necessary for colour demodulation, and a killer detector for discriminating whether a colour signal or a monochrome signal is being received. Outputs of the killer detector are supplied to either the colour control circuit 14 or the demodulators 15, 16, 17 so as to interrupt the function of these circuits, so that colour noises are not produced by them when receiving the mono-chrome TV signals. The reference subcarrier generator causes an automatic phase control function to correctly follow the phase of the input burst signal, generates the subcarriers for demodulation based upon the phase of the burst signal, and supplies them via the transmission paths 22, 23 to the (B-Y) demodulator 15, and the (R-Y) demodulator 17. The demodulators 15 and 17 both deliver positive and negative polarity outputs. For example, they deliver the positive polarity outputs to each of their output terminals 27 and 29 in accordance with the polarity of the picture tube driving circuit connected to the output stages of the demodulators, and also the negative polarity outputs via the transmission paths 25, 26 to a matrix (G-Y) demodulator 16 so as to obtain the (G-Y) output signal at the (G-Y) output terminal 28.
FIG. 2 shows a circuit diagram of the known colour signal processing circuit of the PAL TV signal receiver. Circuit elements having the same functions as in FIG. 1 are indicated by the same numbers. A chroma signal output from the colour control circuit 14 is supplied to a 1 H delay line 31 and to a matrix circuit 33 through an attenuator 32. To this matrix circuit 33 the output from the 1H delay line 31 is also supplied. In the
matrix circuit 33, the chroma signal which is delayed by 1 H (1 horizontal sync period) and the chroma signal which is not delayed are matrixed so as to separate them into the (B-Y) signal component and the (R-Y) signal component. These signal components are supplied to the (B-Y) demodulator 15 and the (R-Y) demodulator 17 respectively. As is known, in the PAL system the modulation axis for the (R-Y) signal component is transmitted by being inverted by 180.degree. every 1 horizontal sync period (=one line period). This is one of the features of the PAL TV system. That is, when a vector synthesizing operation is performed between the delayed chroma signal and the undelayed chroma signal, a subcarrier phase distortion with respect to the demodulation signal may be mitigated. Compared with the NTSC system, the PAL system provides the advantages that, since an adverse influence on the phase distortion is not substantially given to the PAL signal in the transmission paths, the hue control circuit can be omitted and the burst signal separated in the bandpass amplifier 12 can be directly supplied to the colour sync circuit 21 to be used to generate a signal as a reference subcarrier. A subcarrier for the (B-Y) demodulation obtained in the colour sync circuit 21 is supplied to the (B-Y) demodulator 15. A subcarrier for the (R-Y) demodulation is supplied to a PAL switch circuit 34 which is driven by horizontal flyback pulses and inverted every 1 H period, in which the subcarrier is subject to a phase correction. The phase-corrected subcarrier is then supplied to the (R-Y) demodulator 17. The inverting operation of the PAL switch circuit 34 is controlled by the output information of the colour killer which is obtained from a killer detector in the colour sync circuit in such a manner that the subcarrier is in-phase with the transmission signal. When receiving the PAL TV signal, an output of a flip-flop circuit in the PAL switch circuit 34 is inverted and non-inverted by the horizontal flyback pulses, but this inverting operation is interrupted for 1 H period by the so-called ident signal upon receipt for the colour killer signal, with the result that it is controlled to realize the normal phase relation between the subcarrier for the (R-Y) demodulation and the transmission signal.
A common processing circuit may be conceived as shown in FIG. 3 from each independent processing circuit.
In FIG. 3, the same functional circuit elements have the same symbols as in FIGS. 1 and 2. This common processing circuit further comprises a system selection circuit 35 and system selection means 36. The circuit 35 may switch operations of the PAL matrix circuit 33, the PAL switch circuit 34 and the hue control circuit 19 for the PAL or the NTSC system. When receiving the PAL TV signals, the control function of the hue control circuit 19 is interrupted, and the burst signal separated in the bandpass amplifier 12 is introduced into the colour sync circuit 21 without any processing. When receiving the NTSC TV signals, the chroma signal delivered from the colour control circuit 14 is supplied via a part of the
matrix circuit 39 to the (B-Y) demodulator 15 and the (R-Y) demodulator 17, in which the chroma signal is not matrixed. A subcarrier for the (R-Y) demodulation derived from the colour sync circuit 21 is supplied to the (R-Y) demodulator 17 via a part of the PAL switch circuit 34 in which the phase of the signal is not inverted.
As explained above, the signal processing circuit in FIG. 3 switches its signal processing paths in response to the TV systems. According to such a multi-mode signal processing circuit, the system selection circuit 35 may determine whether the chroma signal is matrixed or not and the PAL switch circuit 34 may determine whether the (R-Y) demodulation axis is inverted by 180.degree. or not every 1 H period. That is, the phase processing functions in the colour signal processing circuit are selected in response to the PAL or the NTSC TV signal.
Points of difference between the PAL and the NTSC systems are listed up in TABLE 1.
TABLE 1 ______________________________________
NTSC system system ______________________________________ ##STR1## (Amplitude ratio of demodulation signal components) 1.8 1 ##STR2## (Amplitude ratio of demodulation signal components) 0.6 0.3 (R - Y) - (B - Y) (Phase difference 90.degree. 105.degree. between demodulation signal components) (G - Y) - (B - Y) (Phase difference 240.degree. 240.degree. between demodulation signal components) ______________________________________
TABLE 1 shows that, assuming that the amplitude ratio of demodulation signal components between (B-Y) and (R-Y) is calculated and the resultant ratio is defined as "1", there is a difference between the PAL system and the NTSC system. These ratios are obtained by detecting the amplitudes of the demodulation signal components when transmitting basic colour signals (red, green and blue). Because the reference white colour (=colour temperature) at the transmiter end has a different value in each TV system, a difference exists in the demodulation ratio. Consequently, in the above-mentioned common mode type colour processing circuit, it is also necessary to change the circuit gain in order to realize the desired amplitude ratios as shown in TABLE 1.
Demodulation axes for each signal component of the NTSC and PAL systems will now be explained. The (B-Y) signal component and the (R-Y) signal component are supplied to the (B-Y) demodulator 15 and the (R-Y) demodulator 17 respectively. To these (B-Y) and (R-Y) demodulators 15 and 17, subcarriers for (R-Y), (B-Y) demodulations generated in the colour sync circuit 21 are respectively supplied. When receiving the NTSC signals, a phase difference of 105.degree. is set between the subcarriers for (B-Y) and (R-Y) demodulation (refer to FIG. 4C). On the other hand, a phase difference of 90.degree. is ser between them (refer to FIG. 4b). The subcarrier for (R-Y) demodulation is inverted inphase in every 1 H period. As explained above, these demodulation axes for the (B-Y) and (R-Y) signal components are determined by a subcarrier generated by the colour sync circuit 21. However, as to demodulation of the (G-Y) signal components, the (G-Y) demodulator 16 employing a known matrix circuit is used.
FIG. 4a is a simplified circuit diagram of the (B-Y), (G-Y) and (R-Y) demodulators 15, 16 and 17. In the (B-Y) demodulator 15, numeral 42 is a phase detector utilizing a double balanced differential amplifier, and numeral 41 is a constant current source. A subcarrier for the (B-Y) demodulation (CWB) and a chroma signal (CRO) are applied to the phase detector 42, at the output terminals 42a and 42b of the phase detector 42, the polarities of the detector outputs of which are opposite to each other, i.e., the demodulated outputs for the (B-Y) component are obtained. One of these demodulated outputs is delivered from an output terminal 27 via an output resistor of (B-Y) demodulation 43. The remaining output is applied to the (G-Y) demodulator 16. The (R-Y) demodulator 17 is constructed similar to the (B-Y) demodulator 15, and comprises a phase detector 46 and a constant current source 45. Into this phase detector 46 the subcarrier for the (R-Y) demodulation (CWB) and the chroma signal (CRO) are input. At the output terminals 46a and 46b of the phase detector 46 the detector outputs having opposite polarities, i.e., the (R-Y) demodulated outputs, are obtained. One of these outputs is delivered from the terminal 29 via resistor 47. The remaining output is applied to the (G-Y) demodulator 16. The (G-Y) demodulator 16 is constructed between a power line and a reference ground potential line, there is connected a series circuit of resistor 48, 49 and a constant current source 50. The (B-Y) demodulation output is applied to a junction between these resistors 48 and 49, and the (R-Y) demodulation output is applied to a junction between the resistor 49 and the current source 50. Then the (G-Y) demodulation output is delivered from the output terminal 28 via a resistor 51.
This (G-Y) demodulation output is obtained by matrixing the (B-Y) demodulation output and the (R-Y) demodulation output. Since in the standard television transmission system a predetermined ratio is decided between a luminecent signal (Y) and three primary colour signals (R), (G), (B), the (G-Y) demodulation output may be necessarily defined by the remaining demodulation outputs.
Suppose that demodulation conversion conductances of the phase detectors 15, 17 are GB and GR, respectively. The respective amplitudes of demodulation outputs EB, ER and EG appearing at the output terminals 27, 29 and 28, and DC voltages VB, VR and VG are obtained as follows (ei=input signal). EQU EB=ei.multidot.GB.multidot.R43 (1) EQU VB=VCC-1/2I41.multidot.R43 (2) EQU ER=ei.multidot.GR.multidot.R47 (3) EQU VB=VCC-1/2I45.multidot.R47 (4) EQU EG=ei.multidot.GB.multidot.R48+ei.multidot.GR.multidot.(R48+R49) (5) EQU VG=VCC-1/2I41.multidot.R48-1/2I45.multidot.(R48+R49)-I50(R43+R47) (6)
where R43, R47, R48 and R49 are resistance values of the resistors 43, 47, 48 and 49 respectively, and I41, I45 and I50 are currents flowing through the constant current sources 41, 45 and 50.
If proper values are selected so that the DC voltages VB, VR and VG are equal to each other, there is no DC level variation, nor variation in intensity of the picture screen when changing the PAL system and the NTSC system.
When receiving the PAL system, the DC voltages VB, VR and VG are set equal in their level in the demodulators 15, 16 and 17, and simultaneously EB/ER=1.8 and EG/ER=0.6 are set in order to satisfy the amplitude ratios shown in the TABLE 1. As a result, vectors of the demodulation signal components are demodulated as follows: (R/Y)/(R-Y)=1.8, (G-Y)/(R-Y)=0.6, a 90.degree. phase difference between the (R-Y) and (B-Y) axes, and a 240.degree. phase difference between the (G-Y) and (B-Y) axes.
Then, when the system changes the PAL reception into the NTSC reception, a vector as shown in FIG. 4c is obtained, and it should be noted that since these demodulators 15, 16, 17 are designed to be adapted to the PAL sytem, the amplitude ratios between (B-Y) and (R-Y), and between (G-Y) and (R-Y) are different from those for the NTSC system described in TABLE 1. Furthermore, because, those amplitude ratios are different from the listed values, the phase of the (G-Y) axis is shifted from a formal phase position (denoted by a broken line in the vector diagram). Accordingly, it is necessary to take measures to shift back the phase of the (G-Y) signal component to the formal phase position, and also to correct the amplitude ratios between (B-Y) and (R-Y), and between (G-Y) and (R-Y) to the values listed in the table.
As is well known, the (G-Y) demodulation output may be obtained by vector-synthesizing the (B-Y) and (R-Y) demodulation outputs. Namely, between the luminance signal (Y) and the three primary colour signals (R), (G), (B), the following relation exists: EQU Y=0.30 R+0.59 G+0.11 B
If (Y), (R-Y) and (B-Y) are determined, the (G-Y) demodulation output is obtained based upon the following relation: EQU (G-Y)=-0.51 (R-Y)-0.19 (B-Y)
In designing such a multi-mode type colour TV receiver, there are a lot of problems in the colour signal processing circuit. For example, each of the demodulation axes (R-Y), (B-Y) and (G-Y) must be adapted to each of PAL and NTSC system, and furthremore, adequate values must be selected for the amplitude ratios for (B-Y)/(R-Y) and (G-Y)/(R-Y).
The multi-mode type colour television receiver is constructed to change the internal signal processing models. In this case it is necessary to avoid the signal level variation in accordance with the mode change. Furthermore it is perferable to utilize as many commonly-used circuit elements as possible, which provides both cost reduction and simple circuitry.
Finally, it is absolutely necessary to correct the phases of each signal as precisely as possible.
It is therefore a primary object to provide a circuit arrangement for processing colour television signals in which, by utilizing the subcarrier for the colour killer obtained from the phase-synthesizing circuit and the demodulation axes for the colour signal components in each TV system, the subcarrier which was used in processing colour TV signals of one TV system can be effectively used for processing colour TV signals of any other TV sytem.
It is a second object to provide a circuit arrangement for processing colour television signals in which, when the internal signal processing circuit portions in the PAL matrix circuit art switched into either the
processing mode or the NTSC processing mode, the amplitude ratios of the separated signals are automatically changed in such a manner that the changed amplitude ratios are perferable to process the signals in the latter stage and the demodulation outputs matching each TV system may be obtained.
It is a third object to provide a circuit arrangement for processing colour television signals, in which when the PAL matrix circuit is designed to be fitted to both TV system, signals applied to the demodulators are matrixed to be derived from two output terminals of the PAL matrix circuit in which the transmission characteristic in the PAL matrix circuit is prevented.
It is a fourth object to provide a circuit arrangement for processing colour television signals in which, when the matrixed output signals from the PAL matrix circuit are supplied to the commonly-used (B-Y) and (R-Y) demodulators, the colour signal processing circuit may set the proper (B-Y)/(R-Y) amplitude ratio for both TV systems in the PAL matrix circuit and also prevent a DC level variation with respect to the demodulators when changing the TV system.
It is a fifth object to provide a circuit arrangement for processing colour television signal in which, in connection with the flip-flop circuit whose operation mode is selectable for the PAL/NTSC sytems and the phase changing means in the phase synthesizing circuit is controlable to the flip-flop circuit, the flip-flop circuit is designed to have three level values for its first and second output voltages.
It is a sixth object to provide a circuit arrangement for processing colour television signals in which, when the PAL matrix circuit portion is changed from the PAL system to the NTSC system and vice verse, the colour signal processing circuit may employ as many commonly-used circuit elements as possible, hold a multi-function of the signal transmission and the switching, and further avoid a DC level variation when changing the TV systems.
It is a seventh object to provide a circuit arrangement for processing colour television signals in which, when the demodulation axes are shifted during the demodulation system change, e.g., the (G-Y) axis is shifted during a change of the PAL system into the NTSC system, the colour signal processing circuit having colour demodulators may correct such a phase shift so as to perform the correct colour demodulation for both TV signals.
It is a further object to provide a circuit arrangement for processing colour television signals in which the colour signal processing circuit having the colour demodulation circuits may adjust the demodulation signal levels (B-Y), (R-Y), (G-Y) to proper levels, and the demodulation circuits may be operated with stable DC levels when changing the TV reception systems.
It is a still further object to provide a circuit arrangement for processing colour television signals in which the colour signal processing circuit may provide such a phase synthesizing circuit that a variation in of the transistors constituting the phase synthesizing circuit will have substantially no adverse effect on the phases of the synthesized outputs.